In selecting a mining site for a mineral, such as selection of a drilling site for an oil well, it is known to rely on previously obtained prospecting data. Since the costs of drilling a well (or sinking a mine shaft) are quite high, it is desirable to minimize the number of selections of wrong sites prior to selection of the correct mining site. However, it is also expensive to increase the amount of seismic data to be used in making site selections for mining or drilling. Thus, it is known to use various techniques to increase the signal to noise ratio of seismic traces obtained during prospecting of a target mining area. That is, several methods are known to highlight the information contained in the seismic traces generated as a result of seismic prospecting.
In seismic prospecting as contemplated herein, there is provided a source of energy, typically sonic energy, which is generated at or near the surface of the earth or of a body of water in offshore prospecting In that regard, it will be understood that "sonic energy" or "seismic energy", as used herein, includes all possible types of wave propagation in seawater and in the earth,
As is known, in sonic prospecting the generated energy travels through the earth or water medium to discontinuities and interfaces among various strata thereof. At each such discontinuity or interface, there is provided both a reflected and a diffracted seismic wave. These waves in turn return to the surface and are detected by a plurality of seismic transducers, which may be hydrophones or geophones, for example, arranged along a seismic line having a particular geographic orientation. The magnitude and phase displacement between the returning signals and the originally generated signals are indicative of the displacement between the source of the seismic signals and the reflecting and/or diffracting interface. The data obtained by a plurality of linear arrangements of such transducers may be used to map the various interfaces and discontinuities.
Thus, the various subsurface earth formations, as well as sub-oceanic formations, may be explored for valuable resources in reliance on the seismic prospecting data. It should be noted that many techniques are known for obtaining the prospecting data.
In these techniques, one or more seismic "shots" are generated at one or more "shot points" in any suitable manner, such as by explosion of a dynamite charge, dropping explosive charges or other weights, pulsing an underground or underwater transducer, generating gas explosions, using air guns, using hydraulic thumpers, etc. In each case, seismic energy is generated and transmitted in many directions, including downwardly to a series of geologic strata or formations. As noted above, the energy is reflected by the formations and is detected by one or more detectors along or near the surface, arranged in a predetermined pattern.
Each of the commonly used techniques processes the arriving signals, provided by the detectors, to compute two way travel time for a shotpoint at the detector location, including travel to and from a reflecting interface structure.
In the known approaches, one or a sequence of seismic energy transmissions are initiated by the one or more shot points, the energy reflected by the discontinuities or other subsurface structures being detected by the above described transducers. Seismic prospecting of sub-oceanic structures is frequently accomplished by having an exploration vessel towing a seismic streamer, having plural pressure sensitive detectors or hydrophones thereon. Impulsive sources, such as explosives or air guns, may also be towed by the vessel.
The source transducers are fired and the echoes of the energy thus generated are received by the hydrophones as variations in pressure, are converted to electrical signals, digitized, and are recorded against time (on paper or magnetic media) as seismic traces for later use in interpretation of the subsurface structures and for use in selection of appropriate mining or well drilling sites.
For offshore prospecting, wherein a plurality of geophones may be spaced fairly closely to one another (approximately 82 feet apart, for example) along a single seismic line, the various lines are typically placed on the order of one-half mile to one mile apart from one another. Thus, although return data are relatively dense in a longitudinal direction determined along the seismic lines of geophones, these data are nonetheless relatively sparse in the direction transverse thereto. Any determinations of dip and/or strike in the transverse direction are thus subject to error because of the relatively large transverse discontinuities between transducers.
In order to provide more accurate representation of subsurface geological structures, more detailed sampling of the subsurface areas are required. Such increased detail is available by providing more dense, e.g., three dimensional, seismic coverage over the exploration area, rather than standard two dimensional data collected by individual lines of receivers to provide a profile representation of the surveyed structures. In the three dimensional seismic prospecting process, the seismic lines of transducers are repositioned at substantially closer transverse intervals to one another than in the conventional approach.
By obtaining the more densely and uniformly packed seismic data, the spurious assumption that the surveyed geologic structure is two dimensional is no longer required. Thus, the received data are manipulated and processed as a true three dimensional wave field, in accordance with correct physical principles. With such processing, the received seismic energy is better imaged to reveal the subsurface geology than the two dimensional representations thereof. Additionally, the availability of three dimensional data permits an interpreter to view the surveyed structure in a three dimensional perspective, either through a time-slice of the seismic data or by obtaining a vertical slice at any orientation. Such flexibility in display makes the seismic interpretation easier, faster and more accurate.
However, because of the time consumptive nature of the seismic prospecting process, it is substantially more expensive to obtain such more highly densely packed three dimensional data than it is to obtain the standard (two dimensional) prospecting data. There is accordingly a need in the prior art to provide a method and apparatus for obtaining three dimensional, densely packed, seismic prospecting data in a less expensive manner, thereby to enable more accurate seismic prospecting of geologic structures
More specifically, it is desirable to be able to regrid existing two dimensional data in a manner to provide three dimensional seismic data, thereby enabling large quantities of data to be evaluated quickly and accurately. A number of unsuccessful attempts have been made in the prior art to regrid such two dimensional data with the aid of varying computational techniques. In addition to requiring seismic data obtained by perpendicular seismic lines, a further significant problem with the prior art is the inability of such regridding programs to function for structures which deviate from a very flat configuration between the points mapped by the two dimensional technique. There is thus a need in the prior art to provide a method for regridding two dimensional data to provide a true three dimensional representation of the surveyed structure.
It is accordingly an object of the present invention to overcome the difficulties of the prior art, to obtain two dimensional seismic data, and to derive therefrom three dimensional, or densely packed, seismic data for selection of appropriate mineral mining or oil drilling sites.
Yet another object of the invention is the selection of mining or drilling sites based on interpolated, three dimensional data derived from two dimensional seismic prospecting data.
It is a more specific object of the invention to provide three dimensional seismic prospecting data for location of mining or drilling sites without requiring a more dense and expensive positioning of various geophones for receiving the reflected and diffracted seismic signals.
It is another object of the invention to provide a method and apparatus for obtaining sparse, two dimensional, data representative of a geological structure or surface, converting the sparse data to more accurate and densely packed three dimensional data representative thereof, and for mining for a mineral based on the combined sparse and densely packed data.
It is still another object of the invention to obtain a mathematical representation of the geologic structure being surveyed by performing operations on sparse, two dimensional, seismic data signals representative thereof, and to select appropriate well drilling sites with greater reliability based on the mathematical representation.
An additional object of the invention is the conversion of two dimensional seismic data to three dimensional, more densely packed, data representative of a geologic surface corresponding to the surveyed structure in order to enable selection of the drilling sites with increased reliability.
Still another object of the invention is the provision of data descriptive of a geologic structure, obtainable at any arbitrarily selected point with respect to said structure, from previously obtained two dimensional seismic prospecting data representing said structure and thus to enable selection of the drilling sites with increased reliability.
Yet a further object of the invention is providing an increased accuracy in mine and well selection decisions by relying on data obtainable by a seismic line, disposed at an arbitrary orientation with respect to a geologic surface, from standard two dimensional prospecting data descriptive of the structure.
It is a further object of the invention to convert sparse, two dimensional, seismic data descriptive of a geologic structure to dense, three dimensional, data descriptive of the structure without the necessity of placing additional lines of transducers or of obtaining additional data and thus to reduce the cost of selecting a proper drilling site in order to reduce.
Yet another object of the invention is the regridding three dimensional seismic data from existing two dimensional data representing a geologic structure in order to reduce the cost of selecting a proper drilling site.